The present invention relates to a circuit configuration for a brake system of automotive vehicles comprising an anti-locking control and serving for regulating the brake pressure build-up on the rear wheels during a controlled braking operation, with the aid of which, through a logical operation of signals mirroring the rotating pattern of the individual wheels and/or the driving pattern of the automotive vehicle, the coefficient of friction is permanently determined and the brake pressure on the rear wheels in "normal operation" is regulated according to the select-low control principle.
The accuracy of the control of the brake pressure in response to the rotating pattern of the wheels and the driving pattern of the automotive vehicle during a controlled braking operation will determine the quality and efficiency of an anti-locking system. The time and the amount of the brake pressure delivery are to be so selected as to reliably insure, that, on the one hand, the driving stability and the steering ability of the automotive vehicle at no time are affected which, in the first place, is attainable by a reduction in the brake pressure while, on the other hand, a short stopping distance is achieved. A high skill on the part of the developer is required to satisfy these actually contradictory requirements.
DE-3903180 A1 discloses a circuit configuration according to which the pressure build-up in the individual cycles, during a control, is dependent on the pressure build-up in the preceding cycle, with a distinction being made between a build-up of a steep gradient and a build-up of a flat gradient, and on the duration of the preceding pressure decrease.
The brake pressure on the front wheels, generally, is controlled individually. With respect to the control of the brake pressure on the rear wheels, in the majority of cases, the select-low control principle, generally, is the one which is preferred. According to that method, an identical brake pressure prevails in the rear wheel brakes, the amount and pattern of which is directed by the "low-wheel" while on the "high-wheel" in a control of that type, the brake pressure, in a large number of cases, is lower than theoretically possible which, necessarily, results in a more or less significant extension of the stopping distance. However, the wheel braked at an excessively low rate, (i.e. the high-wheel), substantially contributes to the driving stability.
In the event of a homogeneous friction coefficient, (i.e. with a friction coefficient identical on the right-hand and left-hand sides of the vehicle), and at an approximately identical brake characteristic or an identical dependence of the brake effect of the two rear wheels on the brake pressure, substantially no extension of the stopping distance will occur due to the select-low control. With disc brakes, the characteristics are adequately identical and linear but not necessarily so with drum brakes. In automotive vehicles with a different locking pressure level on the rear axle, which is due, for example, to the use of drum brakes of different contact pressures, to different friction coefficients of the brake pads (which will, of course, also apply to disc brakes), etc., which, virtually, will result, with an identical brake pressure, in different wheel brake torques, a select-low control will permit the theoretically possible brake effect or brake force utilization on one wheel only. The other wheel will not contribute nearly enough to the braking effect. This is especially disadvantageous and no longer acceptable in situations where the brake force share of the rear wheels, compared to the individually controlled front wheels, is or could be high; this especially applies to roads having a low friction coefficient.
According to DE 3815732 A1, in general, in an automotive vehicle having a select-low control on the rear axle, the rotating patterns of the rear wheels are compared during the control and, after criteria have been recognized that are typical for a continual brake torque difference on the rear wheels, a brake pressure adjustment is performed and, hence, an adjustment of the brake torque.